Today's business communications environment consists of two separate network infrastructures: a voice network (such as a private branch exchange (PBX)) characterized by real-time, high-reliability, constant bit-rate (CBR) connections; and a data network (such as a packet network) characterized by high-bandwidth variable bit-rate (VBR) connections. Business needs for simplified maintenance, management, and access to information on diverse networks are forcing the convergence of these networks along with a new class of real-time multimedia networks. Asynchronous transfer mode (ATM) provides a single infrastructure that cost-effectively and flexibly handles both switching and transmission for the traffic types mentioned above (voice, video, and data) for both local-area networks and wide-area networks. The evolving network convergence requires the adaptation of the legacy PBX voice traffic to ATM. Voice telephony over ATM (VTOA) specifications allow adaptation of compressed or uncompressed voice pulse-code modulated (PCM) data streams into streams (virtual circuits) of CBR cells.
An ATM cell, regardless of the traffic it carries, is a packet 53 octets long: 48 octets of payload attached to a 5 octet header. The header contains addressing and management information used to direct the cell from source to destination and to ensure that the negotiated aspects of the traffic-flow through the ATM network are met. CBR traffic is assembled into cell payloads using ATM Adaptation Layer 1 (AAL1). The AAL1 cell constructor layer uses the first octet of the payload for its header and the remaining 47 octets to carry CBR information. ATM cell construction is then completed by attaching the ATM header to the payload.
The PBX provides multiple PCM streams (active channels, or existing calls) to multiple instances of the cell constructor, and the number of PCM streams and cell constructor instances changes as calls are added or removed. Assuming uncompressed-data, the PBX provides each instance of the cell constructor with one PCM octet every 125 microseconds (assuming a standard 8000 Hz sampling rate). It requires 5.875 milliseconds to fill a cell (47 octets * 125 microseconds/octet). Each fully-constructed (mature) cell is queued for transmission. Using a standard OC3c 155.52 MHz fiber optic interface, it takes 2.83 microseconds to transmit a cell. This creates a large variation in cell delay (jitter) through the transmit queue between a cell that enters the queue when it is the only cell to mature during the cell construction period and cells that enter the transmit queue when cells for many (a large fraction) of the channels (PCM streams) in the system mature at the same time. The variations in cell delay range from 2.83 microseconds for one cell maturing in a cell construction period to 682 microseconds for 241 cells maturing during a cell construction period (assuming a PBX that supports up to 241 simultaneous conversations). Such variation in delay is anathema to the concept of constant-bit-rate traffic.